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/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2001-2004 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution, if
 *    any, must include the following acknowledgement:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgement may appear in the software itself,
 *    if and wherever such third-party acknowledgements normally appear.
 *
 * 4. The names "The Jakarta Project", "Commons", and "Apache Software
 *    Foundation" must not be used to endorse or promote products derived
 *    from this software without prior written permission. For written
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 *
 * 5. Products derived from this software may not be called "Apache"
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 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * .
 */
package org.apache.commons.collections;

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;
import java.util.Iterator;

/**
 * 

* An implementation of a Map which has a maximum size and uses a Least Recently Used * algorithm to remove items from the Map when the maximum size is reached and new items are added. *

* *

* A synchronized version can be obtained with: * Collections.synchronizedMap( theMapToSynchronize ) * If it will be accessed by multiple threads, you _must_ synchronize access * to this Map. Even concurrent get(Object) operations produce indeterminate * behaviour. *

* *

* Unlike the Collections 1.0 version, this version of LRUMap does use a true * LRU algorithm. The keys for all gets and puts are moved to the front of * the list. LRUMap is now a subclass of SequencedHashMap, and the "LRU" * key is now equivalent to LRUMap.getFirst(). *

* * @deprecated Moved to map subpackage. Due to be removed in v4.0. * @since Commons Collections 1.0 * @version $Revision: 1.22 $ $Date: 2004/01/14 21:43:04 $ * * @author James Strachan * @author Morgan Delagrange */ public class LRUMap extends SequencedHashMap implements Externalizable { private int maximumSize = 0; /** * Default constructor, primarily for the purpose of * de-externalization. This constructors sets a default * LRU limit of 100 keys, but this value may be overridden * internally as a result of de-externalization. */ public LRUMap() { this( 100 ); } /** * Create a new LRUMap with a maximum capacity of i. * Once i capacity is achieved, subsequent gets * and puts will push keys out of the map. See . * * @param i Maximum capacity of the LRUMap */ public LRUMap(int i) { super( i ); maximumSize = i; } /** *

Get the value for a key from the Map. The key * will be promoted to the Most Recently Used position. * Note that get(Object) operations will modify * the underlying Collection. Calling get(Object) * inside of an iteration over keys, values, etc. is * currently unsupported.

* * @param key Key to retrieve * @return Returns the value. Returns null if the key has a * null value or if the key has no value. */ public Object get(Object key) { if(!containsKey(key)) return null; Object value = remove(key); super.put(key,value); return value; } /** *

Removes the key and its Object from the Map.

* *

(Note: this may result in the "Least Recently Used" * object being removed from the Map. In that case, * the removeLRU() method is called. See javadoc for * removeLRU() for more details.)

* * @param key Key of the Object to add. * @param value Object to add * @return Former value of the key */ public Object put( Object key, Object value ) { int mapSize = size(); Object retval = null; if ( mapSize >= maximumSize ) { // don't retire LRU if you are just // updating an existing key if (!containsKey(key)) { // lets retire the least recently used item in the cache removeLRU(); } } retval = super.put(key,value); return retval; } /** * This method is used internally by the class for * finding and removing the LRU Object. */ protected void removeLRU() { Object key = getFirstKey(); // be sure to call super.get(key), or you're likely to // get infinite promotion recursion Object value = super.get(key); remove(key); processRemovedLRU(key,value); } /** * Subclasses of LRUMap may hook into this method to * provide specialized actions whenever an Object is * automatically removed from the cache. By default, * this method does nothing. * * @param key key that was removed * @param value value of that key (can be null) */ protected void processRemovedLRU(Object key, Object value) { } // Externalizable interface //------------------------------------------------------------------------- public void readExternal( ObjectInput in ) throws IOException, ClassNotFoundException { maximumSize = in.readInt(); int size = in.readInt(); for( int i = 0; i < size; i++ ) { Object key = in.readObject(); Object value = in.readObject(); put(key,value); } } public void writeExternal( ObjectOutput out ) throws IOException { out.writeInt( maximumSize ); out.writeInt( size() ); for( Iterator iterator = keySet().iterator(); iterator.hasNext(); ) { Object key = iterator.next(); out.writeObject( key ); // be sure to call super.get(key), or you're likely to // get infinite promotion recursion Object value = super.get( key ); out.writeObject( value ); } } // Properties //------------------------------------------------------------------------- /** Getter for property maximumSize. * @return Value of property maximumSize. */ public int getMaximumSize() { return maximumSize; } /** Setter for property maximumSize. * @param maximumSize New value of property maximumSize. */ public void setMaximumSize(int maximumSize) { this.maximumSize = maximumSize; while (size() > maximumSize) { removeLRU(); } } // add a serial version uid, so that if we change things in the future // without changing the format, we can still deserialize properly. private static final long serialVersionUID = 2197433140769957051L; }




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